(1) Field of the Invention
The present invention relates to low expansion ceramic materials and a process for producing these ceramic materials. More particularly, the invention relates to zirconyl phosphate-zircon base low expansion ceramic materials having excellent thermal shock resistance and heat resistance, and to a process for producing such ceramic materials.
(2) Related Art Statement
With advances in recent industrial technology, demands for materials having excellent heat resistance and thermal shock resistance have increased. The thermal shock resistance of ceramic materials is influenced by physical properties of the materials such as coefficient of thermal expansion, heat conductivity, strength, modulus of elasticity, Poisson ratio, etc, and is also influenced by sizes and configurations of resulting articles and heating and cooling process, that is, a heat flow speed.
Among these factors influencing the thermal shock resistance, the coefficient of thermal expansion particularly has the greatest contributory percentage. It is known that particularly when the heat flow speed is large, the thermal shock resistance is largely influenced solely by the coefficient of thermal expansion. Under these circumstances, development of low expansion materials having excellent thermal shock resistance have earnestly been demanded.
As relatively low expansion ceramic materials having a coefficient of thermal expansion of around 5 to 20.times.10.sup.-7 (l/C.degree.) in a temperature range from 40.degree. C. to 800.degree. C., cordierite (MAS), lithium-aluminum-silicate (LAS), and the like are known. The melting points of these materials specifically are low, the melting point of cordierite is 1,450.degree. C. and LAS has a melting point of 1,423.degree. C. For instance, in the case of ceramic honeycomb structures used as catalyst substrates for automobile catalyst purifiers, a fitting position of a catalyst converter is changed from a conventional underbed to near an engine to improve catalytic purifying efficiency, or a temperature of exhaust gases increases due to design changes, for instance, attachment of a turbocharger to improve fuel mileage and engine output. Consequently, since a temperature of catalyst beds increases by 100.degree. to 200.degree. C., it has been discovered that even a cordierite base honeycomb structural substrate having a high melting point may clog due to melting. Accordingly, development of low expansion materials having thermal shock resistance equivalent to or superior to that of cordierite as well as excellent heat resistance have strongly been desired.
As ceramic materials having relatively low thermal expansion and high heat resistance, mullite (3Al.sub.2 O.sub.3.2SiO.sub.2, coefficient of thermal expansion: 53.times.10.sup.-7 /.degree.C., melting point: 1,750.degree. C.) and zircon (ZrO.sub.2.SiO.sub.2, coefficient of thermal expansion: 42.times.10.sup.-7 /.degree.C., melting point: 1,720.degree. C.) are available. However, they have drawbacks that their coefficients of thermal expansion are both high and consequently their thermal shock resistance is low.
As known examples of low expansion ceramic materials mainly consisting of zirconyl phosphate, there are high strength zirconyl phosphate sintered bodies disclosed in Japanese patent publication No. 61-12,867 which each contain 2 to 10 mol % of a mixture of SiO.sub.2 /Nb.sub.2 O.sub.5 at a mixing molar ratio of 1 to 8 and 1 to 6 mol% of Al.sub.2 O.sub.3 ; low expansion zirconium phosphate porcelains disclosed in Japanese patent application laid-open No. 61-21,853 which each contain 0.5 to 0.6% by weight of magnesium phosphate as a sintering aid; low thermal expansion zirconyl phosphate ceramic materials disclosed in Japanese patent application Laid-open No. 61-219,753 which contain at least one kind of a material selected from the group consisting of AnO, MgO, Bi.sub.2 O.sub.3, MnO.sub.2, Co.sub.2 O.sub.3, NiO, TiO.sub.2, CeO.sub.2, Nb.sub.2 O.sub.5 and Ta.sub.2 O.sub.5 as a sintering aid and a group consisting of SiO.sub.2 and a silicate as a grain growth retardant in a total amount of0.3 to 10% by weight, an amount o f said at least one kind of the material from each of the groups being not less than 0.1% by weight; zirconium phosphate ceramic materials disclosed in Ceramic Technology Study Annual Report No. 9, pp 23 to 30 (1982) published by Nogoya Institute of Technology, which each contain 2% by weight of an additive such as MgO, MnO.sub.2, Fe.sub.2 O.sub.3, ZnO, etc. However, none of the above ceramic materials contain zircon as a secondary phase and their heat resistance is not good because their sintering mechanism is a liquid phase sintering utilizing production of a liquid phase having a low melting point. Thus, such techniques could not meet the above-mentioned demands.